1. Field of the Invention
The present invention relates to a high-performance collimator and compact spectrophotometer capable of performing spectrophotometry without using a prism or diffraction grating.
2. Description of the Related Art
Conventionally, a method for measuring spectral intensity by guiding light in a different direction for each wavelength using a prism or a diffraction grating, irradiating the light to a linear sensor or the like, and measuring the output from the elements of the linear sensor was generally used to perform spectrophotometry. However, a certain amount of space is needed to separate and guide the diffracted light in different directions according to wavelength when a prism, diffraction grating, or the like is used. An unacceptably large spectrophotometer thus resulted. Other drawbacks included the fact that the light intensity decreased due to the measured light being passed through a slit when guided to the prism or diffraction grating, and that high-speed measurement was difficult to accomplish because the storage time required for the linear sensor was considerable.
Several methods featuring a linear variable filter (occasionally referred to hereafter as LVF) have been suggested and developed with the aim of overcoming the abovementioned drawbacks. For example, the technique disclosed in Japanese Patent Application Laid-open No. H5-322653, and the technique disclosed in U.S. Pat. No. 5,872,655 are widely known, and a spectrophotometer that uses a different system of linear variable filter is disclosed in U.S. Pat. No. 6,057,925 and is commercially available. In this techniques, diffracted light emitted from a linear variable filter is focused on a linear sensor by inserting an optical system for an erect same-size image between the linear variable filter and linear sensor, and a GRIN (Gradient Index) lens or a Micro Lens Array is used as a compact focusing system for the erect same-size image.
The following problems nonetheless exist in the system disclosed in Japanese Patent Application Laid-open No. H5-322653, and in the method disclosed in U.S. Pat. No. 5,872,655. Specifically, multiple reflections occur between the linear variable filter and linear sensor due to these methods being configured such that the linear variable filter is attached to the linear sensor, and the spectral characteristics thus deteriorate.
While these problems are eliminated in the method disclosed in U.S. Pat. No. 6,057,925, other problems nonetheless occur. Specifically, the GRIN lens comprises a total of 28 cylindrical lenses disposed in two rows. Consequently, when a surface image similar to a linear variable filter is projected, exactly 28 peaked irregularities occur in the output of the linear sensor because a composite image made by 28 cylindrical lenses is focused on the linear sensor. The dimensional accuracy of the spectral wavelength output will thereby decrease even when the positional accuracy of the spectral wavelength is enhanced.
Meanwhile, mechanical collimators are conventionally used for transmitting light in a parallel fashion. Specifically, a large-scale collimator is used in large-scale optical systems such as edge locators and width meters for web-shaped measurement objects. This kind of mechanical collimator may also be used in the transmission of light between a linear variable filter and a linear sensor. However, such collimators have bulky structures, and it was believed to be impossible to manufacture a compact product that satisfies high resolution requirements and is capable of being used in transmission of light between a linear variable filter and a linear sensor.
Aiming to develop a method for overcoming the aforementioned problems, the inventors have succeeded in obtaining spectral results having high wavelength resolution and devoid of any irregularities by placing a fiber optic sheet (hereafter abbreviated as FOP) between the linear variable filter and the linear sensor, or at the forward surface of the linear variable filter, and have filed for a patent as Patent Application 2001-078176 (hereafter referred to as “the prior application”). An FOP having high directivity (NA=0.35) is used especially for enhancing wavelength resolution in the embodiments of this invention.
A structural diagram thereof is depicted in FIG. 16. Fiber optic sheets 33a and 33b are provided above the sensor package 32 of a linear sensor 31 in the manner shown in the figure, and a linear variable filter 34 is bonded to the fiber optic sheet 33a on the top thereof. The space between the linear sensor 31 and the fiber optic sheet 33b is filled with a transparent resin 35, and is approximately 0.01 mm wide. The numerical aperture (NA) of the fiber optic sheets 33a and 33b is assumed to be 1 in this embodiment.
The reason that the fiber optic sheets 33a and 33b are divided into two layers is that the fiber optic sheet 33a is used instead of the surface cover glass of the sensor package 32 of the linear sensor 31; and when this is unnecessary, a single fiber optic sheet may be used, or 33a and 33b may be formed in an integrated manner.
For light entering the linear variable filter 34 from the upper portion of the figure, only light with a wavelength determined by the entry location thereof in the linear variable filter 34 is transmitted, spectrally divided according to the location of the linear variable filter 34, guided by the fiber optic sheets 33a and 33b, and directed through the transparent resin 35 to the corresponding pixel of the linear sensor 31. Spectral measurement can thus be performed by processing the output of each pixel of the linear sensor 31.
Because the linear variable filter 34 and fiber optic sheet 33a are attached, there is no light diffusion in the space therebetween, but a slight degree of light diffusion still occurs between the fiber optic sheet 33b and the linear sensor 31 even if the numerical aperture of the fiber optic sheets 33a and 33b is equal to 1. However, this does not present much of a problem, since the interval thereof is approximately 0.01 mm. The light transmission rate from the linear variable filter 4 to the linear sensor 1 is approximately 60–70% in this embodiment, which is apparently by no means inferior when compared with the conventional example, in which a linear variable filter and linear sensor are adjacent.
Problems nonetheless exist in this method as well. Specifically, the effective waveband of the FOP in which the actual NA is 0.35, ranges only from 400 to 800 nm. Consequently, high-resolution spectral results cannot be obtained for longer wavelengths (800 nm or greater). At the same time, it is said that linear variable filters used for wave length from 0.4 μm to 20 μm can be manufactured
To use an FOP at greater wavelengths, a fiber for the waveband thereof must be newly manufactured. It is costly and impractical to make an optical fiber corresponding to each wavelength. Another drawback is that light transmissivity is attenuated when NA=0.35, even in a visible-range FOP.